Traveling wave tube



Inventor:

His Attorney.

Feb; 18, 1958 c; M. BRANCH, 'JR

TRAVELING WAVE TUBE Filed March 3, 1953 Garland M. Branchflr: by 22,! 4. 7

nited States Patent "I ti" 2,824,257 Patented Feb. 18, 1958 WAY Gar-lamb M.. Branch, 'Jr.',. Schenectady; N. Y;, assignorto GeneralElectrio-Gompany, a corporation of. New. York.

Application March '3, 1953, seriarNo. 340 0.74

My invention relates toiimprovements in electron dis.- charge devices of theztype generally knownastraveling wave tubes.

In traveling wave tubes: energy is exchanged between an electron stream and a propagated wave, the average velocity-of the electron stream usually being somewhat greater than that of the propagated wave in orderto transfer energy thereto. The structure which transmits the traveling wave i's'usually a helical conductor so that the velocity of the wave alongthe helix axis is substantially less than the actual velocity along the conductor itself and in the vicinity of conveniently obtainable electron beam velocities. For reasonable gain and ctficiencies at a given beam current density, the diameter of the helix should not exceed a certain maximum value relative to the wave length of the propagated wave. Consequently; in the progress of the art towards higher and higherfrequencies; the relatively small diameter helix dimensions requiredintroduce problems of alignment, construction, and} heat; dissipation.

It is, therefore, an object of my invention to provide an improved traveling wave tube having higher gain and efficiency at higher frequencies, i

It is another object. of my invention to. provide a helical wave transmitting structure having relatively large physical size for a. given operating frequency.

Conversely, it is an object of my. invention to provide a helical transmitting structure. for a. traveling wave tube permitting higher frequency operation, for a given. phsYi; cal size of the structur According to one. aspect of my invention, a. traveling wave tube is provided. with a slow wave; transmitting structure comprising two concentric helical conductors having ditferent diameters to accommodate a beam. of electrons in the annular gap between the coaxial. helices. The helices. are. coupled together at, each end; and a wave propagated alongthe composite structure is. amplified; by the. electron beam The. frequency of. operation depends primarily on the width of the annular gap betweenthe two helices rather thanon the overall diameter of either helix, thus. releasing the. helix dimensions from the restrictions'. imposed by the desired: operatingfrequency upon the size of the conventional single helix transmitting structure. The total: beam currentavailable between the double helicesis also substantially greater'than that pos sible in the case oftheconventional single helix traveling wave tube, resulting infurther' gainand increased efiiciency at higher power, levels.

The features which are. desired to protect herein are pointed out with particularity in the appended claims. The invention itselftogether with further objects and advantages, thereof may best be understood by reference to the following description taken, in connection with the drawings. in which Fig, 1 represents a schematic. longitudinal. section. or a traveling. wa tu e e dying, the principles. at. my invention; and 'Fig. 2 is. a schematic longitudinal section of another-- traveling: wave tube embodying my invention. I

Referringnowto Fig. 1, the traveling wave tube; shown therein. amplifies an electromagnetic wave propagated between input terminal 1- and output terminal 2. Connectedi between these terminals is a double helix structure which transmits the electromagnetic waves at a relatively low velocity commensurate with the velocity range conventionally obtainable for electron beams. This structuremay suitably comprise an inner conductive wire helix 3 and an outer coaxial conductive wire helix 4 of larger diameter, the end turns of the helices being connected in parallel to the terminals 1 and 2. The con? ductors are shown as round wires having: a diameter which is very small compared to. the helix. diameters. Other conductor cross sections maybe alternatively employed, such as in conductive ribbons, for example. In accordance with my invention, the helix diameters are sufliciently difierent to provide an annular space. betweenthem for the passage of one or more electron beams. As shown. in the drawing, the diameter of the outside helix may suitably be one and one-half times that of the inside inner helixto provide the desired annular spacing;

Near theinput'termi'nal atthe input end of the. helix as.-

sembly-is'anelec'tron gun array for providing alargeelectron current in the form of'a plurality of. electron. beams along the annular space between the inner and" outer helices 3 and. 4 in a direction parallel to the axis of. the helix assembly. Two of six regularly spaced. guns 5 are illustrated. in Fig. 1, each gun comprising-a. cathode 6,v a cathode heater 7, and a control or focusing electrode. 8. For convenience, the corresponding electrodes ofthevarious guns are. connected in parallel. At the other end of the helix structure beyond the output terminal. 2 is, provided" an anode or collector electrode 9 whichmay suitably take the shape of a cylinder having. an. end surface. thereof facing the annular space between the twohelices to collect the electrons from the electron guns. The number ofguns. is. not critical, each beam comprising in. efiect a segment. of. a hollow annular beam.

Enclosing the helix assembly is a tubular envelopecomprising an inner glass tube 10. on which the inner helix 3 is supported and anouter glass. tube 11 which closely surrounds the. outer helix 3. These inner. and outer tubular envelope members are sealed. together at the input end. of the. traveling wave tube. to enclose. the electron guns, the.

- inner tube 10 being thus supported by the outer tube 11.

At the output, end inner tubing member 10 is. closed, and the end of. the outer tubing member 11 is hermetically secured to. the collector 9. The annular space betwenthe. inner and. outer tubing members is evacuated. in the cus-v tomary manner. The electron gun structure is provided with relatively stiff conductive leads which are sealedthrough. the. glass envelope so formed to. both provide external. terminals and support the gun structure. The. envelope structure illustrated is particularly adapted to sup.- port aconcentric helix structure since the inner and,- outer helicesmay be. respectively aligned. and supportcdson. the facing surfacesof the inner and. outer glass tubes.

Since. the. collector electrode 9 forms part of the envelope. its outer surface is exposed to. the. air to, facilitate dissipation of. heat caused by the. collection ofthe large. electron current permitted by the double. helix traveling wave tube structure. Additional collectorcooling means. may alsobe attached, such as the air-cooled radiator. 12 shown. in Fig. 1.

Any suitable means known in the. art for preventing radiation. of. the Wave propagated between the. inputantl output. terminals 1. and. 2 may be employed-.. As shown in Fig. 1' such a means may suitably take the. form of 3 a non-magnetic conductive cylindrical sleeve 13 surrounding the length of the outer glass tube 10. This shield is apertured near the terminals 1 and 2, such an arrangement being expressly adapted for concentric line coupling to external circuits.

As shown in Fig. 1, an input. concentric conductor transmission line section 14 may be utiilzed with its outer conductor connected to theshield 13 and its inner conductor connected to 'the terminal 1. An output transmission line section 15 is similarly arranged with respect to the output terminal 2. Various means of coupling the traveling wave helices to external circuits may be substituted as desired, such as connecting each helix separately to external concentric conductor transmission lines with each pair of lines connected to the signal source on the load by a phase-shifting and power-divid: ing network. I

To provide direct current connections to the helix assembly Without interfering with the propagation of the traveling wave, a quarter wave short-circuiting stub 16 is employed on the concentric line coupling system. Such a stub is one-quarter wave length long at the desired operating frequency or central frequency of the band of operating frequencies. As shown in Fig. 1, the stub has its inner and outer conductors respecitvely coupled at one end to the inner and outer conductors of the coupling section 14 and short-circnited together at the other end. This permits application of the same direct current potential to the shield and helix in order to maintain the generally desired field free region in the interaction space.

Any conventional means for stabilizing the operation of the tube as an amplifier may be employed. To this end attenuation of backward traveling reflected waves may be effected by coating the outer surface of the outer glass envelope 11 with aquadag in a manner Well-known in the art.

In operation, a source of heater voltage 17 is connected to the heater terminals to maintain the emitting surfaces of the cathodes at a sufficiently high temperature to provide high density electron beams. A source of unidirectional voltage 18, conventionally represented as a battery having a voltage dividing resistor connected across its terminals, is connected to maintain the cathodes at a negative potential with respect to ground, that potential being in the order of a thousand or thousands of volts. The helix assembly and the collector electrode 9 are conveniently placed at a positive potential with respect to the cathode by connecting the shield 13 and the collector to ground. The accelerating potential which determines the electron beam velocity when it enters the annular space between helices is applied between the cathodes 5 of the electron guns and the end turns of the helices facing the electron guns at the input end of the tube. Further changes in the beam velocity are subsequently due to the interaction with traveling waves propagated along the annular space between the helices. The magnitude of the beam current is preferably separately controlled and for this reason the space charge control electrodes 8 of the electron guns 5 are connected to an intermediate tap on the voltage source 18.

Focusing of the electron beams throughout their axial path along the annular interaction space is further maintained by an axially aligned static magnetic field within that space. A solenoid winding 19 for producing such a field surrounds the outer shield 13 along its length for this purpose. To simplify the drawing the connection of the solenoid terminals to a unidirectional current source is not shown.

As is well-known in the traveling wave tube art pertaining to conventional single helix tubes, the ratio of pitch to the diameter of the helix and the electron accelerating potential are so related that the electron beam travels in the same direction as the electromagnetic waves propagated from the input to the output terminal of the helix structure with a velocity relationship such that energy is transferred from the beam to the wave. The velocity of the beam is commonly somewhat greater than at least one component of the electromagnetic wave. Under these conditions it is known that interaction between the beam and the wave occurs to amplify the wave as it travels along the helix. One explanation of interaction by J. R. Pierce may be found in the Bell System Technical Journal, vol. 29, No. 1 (January 1950), pp. 6-19. The energy exchange'is available at several beam velocities, and the velocity chosen depends upon a number of factors, including the magnitude of the beam current.

To maintain the maximum coupling between the two coaxial helices in the present invention, the invention is preferably provided with equal pitch angles but are wound in the opposite sense. The pitch itself or turn to turn spacing varies with the helix diameters. With the three to two diameter ratio of the outer to inner helix as shown in Fig. 1, there are three turns of the small helix 3 for every two turns of the larger helix 4. Such an arrangement provides an axial field for one fundamental mode of transmission which is of high magnitude everywhere across the annular gap between the helices were wide frequency range. In this particular construction the current flowing in the outer helix should ideally be'in phase with'and about 1.18 times larger than the inner helix current for "most efficient operation. This can be achieved by separately coupling the inner and outer helices to the signal source and to the load with coupling networks arranged to maintain the desired relative amplitude and phase; With the parallel input and output connections provided with the terminals 1 and 2 of Fig. 1, approximately equal currents are excited substantially in phase in the inner and outer helices. In such a case, a second undesired fundamental mode, although nearly synchronous with the first mode, exhibits a small, non-uniform axial field distribution in the annular gap between the helices and fields comparable in magnitude with those of the first mode only in some of the remaining radial regions. These conditions permit optimum coupling in the one desired mode between a hollow beam of electrons moving down the annular gap and the traveling electromagnetic wave. Each of the six beamsprovided by the electron guns 5 may be considered as a portion of the hollow beam, their effect being much the same as" a continuous hollow beam except that the lower, current results in less gain and power. The second mode, even if excited, has very little interaction with theelectron beam and any attenuating means placed outside. the annular gap to stabilize the tube operating in the first mode willbe equally effective in damping out any transmission in the second mode, the phase velocities in the twomodes being nearly equal.

A major advantage of the double helix traveling wave tube not found in the single helix conventional type flows from the fact that the optimum operating frequency appears to depend primarily on the width of the annular gap between two helices rather than on the over-all diameter of either helix. A useful parameter in specifying the relative dimensions of the two helices and for comparing the double helix structure with the conventionalsingle helix type is the number n, which measures the diameter of the inner helix in units of the annular gap width. Accordingly,

where a and b are the radii of the inner and outer helices 3 and 4, respectively. The parameter n, therefore, is a scale factor indicating the magnification of the smaller helix in the double helix tube over the helix of a conventional tube operating at approximately the same frequency. Overa wide range the optimum operating frequency dependsprimarily upon the width of the an- -7 nular gap between the two helicesv rather than onthe overall diameter of the tube. For example, the operating frequency is theoretically constant within 2 percent when the inner helix diameter ranges from 4 to 10 times a given annular gap width. The various: operating characteristics of double helix tubes for agiven gap width are thus nearly independent of the over-all tube. size, the circumference of the outer helix being ultimately limited in that it should preferably be small compared to a wavelength in free space at" the operating frequency.

Accordingly, for a given radius of a single helix tube and of the inner helix of a double helix tube, the maximum gain may be obtained at a frequency several times higher. Conversely, for optimum operation at a given frequency, the double helix structure has an available range of radial dimensions several times larger than a single helix tube. This results in a significant decrease in the difliculty of constructing high power broad band amplifiers at very high frequency since the problems of alignment, construction, and heat dissipation at high frequencies where very small single helix tube dimensions are required are greatly reduced by the larger and more rugged double helix structure.

Referring now to Fig. 2, another traveling wave tube embodying my invention is illustrated, this tube being especially adapted for higher gain and higher output levels.

Here again is provided a traveling wave tube which amplifies an electromagnetic wave propagated between an input terminal 20 and an output terminal 21. Between the terminals a pair of concentric helical conduetors are connected in parallel, the inner helix 22 and the outer helix 23 correspond respectively to the inner and outer helices 3 and 4 of Fig. 1. While the ends of the respective helices are connected, as by welding, to the input and output terminals as in Fig. 1, further means supporting the helices relative to each other is provided by a number of radially aligned conductive members or pins 24, each of them bridging the annular gap and having its ends secured to the respective helices. With the helix construction shown wherein the helices have an opposite winding direction but equal pitch angles there are three turns of the inner helix for every two turns of the outer helix. Accordingly, in a plane passing through one parallel axis of helices the axis of the tube and through the input and output connections a bridging pin 24 extends radially between each second turn of the outer helix to each third turn of the inner helix. The bridging pins 24 short-circuit the helices together so that it acts not only as a slow wave structure but also as a filter circuit having, for a traveling wave tube, a relatively narrow band pass characteristic. In an actual apparatus corresponding to that described, the bandwidth was approximately six hundred to eleven hundred megacycles. Since this is all the bandwidth desired for many applications, the attenuation of any undesired or reflected waves exceeding that bandwidth is increased.

With the substantially self-supporting concentric helix arrangement of Fig. 2, a conventional single glass tubular envelope 25 surrounding the entire helix assembly is suitably employed. As a result, since no relatively lossy dielectric such as an inner glass tube need be employed, the traveling wave tube may be employed at higher frequencies without any decrease in efficiency due to losses introduced by the inner helix supporting means.

For increased gain and power output a greater total beam current is desirable. Accordingly, an electron gun 26 for producing a hollow electron beam between the inner and outer helices is provided at the input end of the tube. This gun suitably comprises an annular cathode trough 27, suitably made of metal, having its open face directed toward the annular space between the helices. The inner surfaces of the trough are provided with thermionic emitting material and a coiled circular heater 28 is positioned invthe trough for heating the cathode to. the. desired-temperature. A focusing or control electrode 29 comprising. a pair. of concentric cylindrical conductors positioned on either side of the trough completes the electron. gun, the cylinders of. the focusing electrode beingpreferably connected together at a region behind the trough. A collector electrode 30 is positioned opposite the other endof the composite helix'structure and potentials are applied in the same manner as for the circularbeam electronguns'of Fig. 1. It is obvious that other electron gun structures for providing a beam between the helices may be provided without departing from the spirit of my invention.

Since the other portions of the apparatus such as the shield, coupling means, and cooling means may suitably correspond to those of Fig. 1 or may be varied in known manner to meet the frequency and power requirements of the particular installation involved they are not further shown in Fig. 2.

While the present invention has been described by reference to particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the invention. 1, therefore, aim in the appended claims to cover all such equivalent variations as come within the true spirit and scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A slow wave transmitting structure comprising an inner conductive helix and coextensive outer conductive helix of a larger diameter concentric therewith, said outer helix having the same but oppositely directed helix pitch angle, conductive means of limited circumferential extent positioned between and along the length of said helices coupling the helices in parallel, said means comprising radial segments connected between selected facing turns of said helices.

2. Apparatus for transmitting an electromagnetic wave at a velocity less than the velocity of light comprising an input terminal, an output terminal, a pair of coextensive concentric helical conductors of different helix diameters connected in parallel between said terminals, and at least one intermediate conductive connection between facing portions of said pair of helical conductors for providing mutual support for said helical conductors and determining the electrical propagation characteristics of said helical conductors.

3. A traveling wave tube comprising an input terminal, an output terminal, a wave transmitting structure for transmitting electromagnetic waves therealong at low velocity coupled between said terminals which includes an inner conductive helix and coextensive outer conductive helix of a larger diameter concentric therewith, said outer helix having the same but oppositely directed helix pitch angle, and a plurality of conductive members positioned along the length of said helices coupling the helices in parallel said members being radial segments connected between selected facing turns of said helices, and means for providing an electron beam along said structure in the space between said helices at a relative velocity with respect to the velocity of said waves whereby interaction occurs therebetween.

4. A traveling wave tube comprising an input terminal, an output terminal, a wave transmitting structure for transmitting electromagnetic waves therealong at low velocity coupled between said terminals which includes an inner conductive helix and coextensive outer conductive helix of a larger diameter concentric therewith, said outer helix having the same but oppositely directed helix pitch angle, and conductive means positioned along the length of said helices coupling the helices in parallel, said means comprising radial segments connected between selected facing turns of said helices, and means for providing a hollow cylindrical electron beam along said structure in 7 a 7 8 the space between said helices at a velocity slightly greater 2,7 07,759 Pierce May 3, 1955 than the velocity of said waves whereby interaction occurs 2,725,499 Field Nov. 29, 1955 therebetween to amplify said waves. FOREIGN PATENTS References Cited in the file of thisp atent 5 987,303 France Apr. 1;, 1951 993,156 7 France July 1 1951 2 489 082 Z E SEATES 22 1949 668,017 Great Britain Mar. 12, 1952 e ores 0v. r 2,584,308 Tiley Feb. 5, 1952 OTHER REFERENCES 2,588,832 Hansell Mar. 11, 1952. 10 Pierce: Travelling Wave Tubes (1950), pages 44 t0 .7. 2,679,019 Lindenblad May 18, 195 r 

